SUMMER holidays in southern Europe are famous for sun, sea and sex. But a fourth 's' - for siesta - invariably serves to interrupt the fun. Just as you're ready for a post-prandial stroll around the shops, the shutters come down, the locals take to their beds, and life comes to a total standstill.

But these somnolent Southerners know what's good for them, according to Professor Arthur Crisp of the Sleep Disorders Clinic at St George's Hospital, London. 'We begin and end our lives with a sleep in the middle of the day,' Professor Crisp says. 'It may be a contrivance to shrink it to the hours of darkness. In the North we have adapted, but that does not mean it is our natural state.'

There is much to suggest that we are biologically programmed to sleep in the early afternoon. Body temperature drops between 2pm and 4pm, as it does at night, and brain waves during that time follow a nocturnal rhythm. Concentration invariably takes a dive. After around seven hours awake, the body is also thought to produce delta-sleep inducing peptide, a naturally occurring soporific chemical.

It is tempting to suggest that early afternoon sleepiness is merely the result of lunch, but psychological studies have proved otherwise. Dr Andrew Smith, a psychologist at the University of Wales, in Cardiff, has extensively investigated the effects of lunch. His research has shown that the nutritional content or size of the meal has only a marginal bearing on the scale of the 'post-lunch dip'.

'Personality is a bigger factor. Relaxed extroverts tend to be more affected than neurotic introverts. But it's a pretty universal phenomenon. You can either fight it with coffee, exercise or a breath of fresh air, or you can give in and sleep,' Dr Smith says.

If only it were that easy. It is not only Nordic workaholics who frown upon sleeping in the afternoon. Most doctors advise patients who complain of restless nights against siestas. They are seen as the perfect recipe for a bad night. A long sleep at night is universally considered desirable and normal.

However, for the millions of Britons who visit their GPs complaining of fatigue rather than insomnia, a siesta may be the answer. It is not the length of the sleep but the type of sleep that matters.

Medical convention has it that there are five stages of sleep, from drowsiness through to deep sleep, ending with a period of shallow sleep. The cycle is repeated throughout the night. The shallow type of sleep is known as rapid eye movement (REM) sleep, and the deeper sleep is called slow-wave sleep, because brain waves slow down from 8.5 to less than two cycles per second.

The first half of a night's sleep is dominated by moderate to deep sleep, and as the night progresses the amount of shallow REM sleep increases. It is during deep sleep that specific hormones essential for tissue renewal and immune strength are released. By-products of muscle metabolism are eliminated, refreshing the system to start anew. It requires only five hours for this process to take place. It is estimated that the brain takes the same time to recoup.

An afternoon snooze is more likely to be the deeper sort of sleep, providing proportionately more opportunity for recovery, says Professor Ian Hindmarch, founder of the British Sleep Society and head of the sleep laboratory at Surrey University, in Guildford.

Southern Europeans who split their sleep between five or six hours at night and two in the afternoon may therefore be accruing double benefits. Not only do they avoid hours of low-quality sleep at night, they also benefit from high-quality sleep in the afternoon. Maybe this is why, according to a survey on European lifestyles conducted last November, the Greeks spend the least time in bed and the Belgians spend the most.

Those contemplating a switch to the siesta should allow enough time for it. Nothing is worse than waking from an afternoon sleep feeling groggy and disoriented. This happens because a deep sleep has been interrupted. A single cycle of sleep, made up of some or all of the various stages, lasts about 90 minutes. We are most likely to wake up - and to wake easily - between cycles.

'In the Army the alternate periods of sleep and vigilance are two hours, as that is thought to be the maximum one can stay alert and is long enough to gain from sleep,' says Professor Hindmarch. It may be no coincidence that this is the length of the average siesta.

Certainly, not resting in the afternoon can have drastic consequences. Pierre Portero, director of research at the Biomedical Centre for Life and Sport, in France, says that industrial and road accidents increase between 2pm and 4pm. These are mainly accidents of omission, which suggests that the person is not fully alert. Studies at Mr Portero's centre have shown that workers are more efficient after a midday nap.

'The more tired you are generally, the more of a slump you experience in the early afternoon,' says Mr Portero. Grand Prix mechanics, who often work late into the night and may get little slow-wave sleep, were obliged to take a mid-afternoon rest after studies showed that many were half-asleep on the job.

Such an idea is unlikely to impress Anglo-Saxon captains of industry, and, in the absence of a two-hour social hiatus, catnaps are the solution. Keen practitioners have included such prodigiously energetic leaders as Sir Winston Churchill, Baroness Thatcher and Bertrand Russell, all of whom claimed to spend but four or five hours asleep at night.

'If you need it, in a nap you may go straight into a deep sleep,' says Mr Portero, whose studies on sailors in long-distance races show that with regular catnaps of 30-40 minutes they functioned as well as ever.

While his compatriots dozed after lunch, Salvador Dali relied on what he called 'microsleeps' during the day, resting in an armchair and holding a spoon over a bucket - so that soon after falling asleep he would be woken up with a bang.

'The polyphasic pattern of sleep - where it is split up into little chunks - was the norm for our ancestors, who had to keep alert because of animals and other dangers,' says Mr Portero. 'The chronobiological mechanisms are still there. It is just a question of habit.'

With this strong biological drive, it takes less than you may think to get into the habit. Research by Dr Jim Horne, at the Sleep Research Laboratory at Loughborough University, Leicestershire, suggests that 'night owls' adapt more quickly to new patterns of sleep than 'larks'. So if you are the early-to- bed, early-to-rise type, you may need a little help.

If you are unable to snatch a sleep just anywhere, Professor Hindmarch says, it may help to go to bed. It is easier to sleep soundly if your spine is supported and you are comfortable.

Sleep experts may be in the dark about why different patterns seem to suit different individuals, but they are certain that the brain is at its most alert during the early morning and late afternoon. It seems sensible, therefore, for the British, who have the longest working day in Europe, to use the slump in the middle of the afternoon to sleep.